Merge "DO NOT MERGE - MPEG4Extractor: Check mLastTrack before dereferencing." into lmp-dev am: b6ba3d7e47  -s ours am: 37e3ea354a am: ea136eb981  -s ours am: 2456e12801
am: fe97b39795

Change-Id: I4d2a011145b4660d078e2e7d47ffb0fc2489a49a